1. Field of the Invention
The present invention relates to the driving of core samplers or probes into the earth for obtaining data in mineral exploration, scientific research, etc., more particularly, the present invention relates to the driving of said devices into bottom formations of deep lakes and oceans.
2. General Background
Much information concerning the geological past of a given location is contained in the sequences of layers of sediments laid down on the bottoms of lakes and oceans. A considerable portion of that information can be obtained if relatively undisturbed cores of that bottom sediment can be retrieved, showing the local progression of sediment types and co-deposited biological specimens, plus scientifically derivable information on specific mineralization, rates of deposition, etc. In areas in which much of the above is already well established, considerable data, especially that related to thickness of various depositions, may be obtained by an instrumented probe, with no core collection required.
One common way of operating both instrumented probe and core sampling devices (hereafter collectively referred to as "probe") is thrusting a long, relatively slim, device into the earth to at least the depth under investigation. Predrilling to facilitate insertion of the probe is not advisable since an object is either to obtain an undisturbed core or a clear signal of transition from one deposition layer to the next, both of which could be destroyed by pre-drilling and its resultant mixing of materials from all levels, spread along and pressed into the walls of the hole.
Therefore, the probe may be driven into undisturbed bottom formations by one of the following methods. The probe may be inserted by hand if it is small, short, and the material is not too hard. The probe may also be driven in by a hammering action. The probe may also be inserted by some heavy mechanical means such as a hydraulic or jacking device. Note that the second and third methods may be accomplished in relatively shallow water by energizing devices on the surface acting on the top of probes long enough to reach the desired depth. Of course, a depth limit is reached in which the slender unsupported section of the probe in the water column will bend rather than penetrate. At such greater depths, the unit must be self-contained, in that the driving unit accompanies the probe to the bottom. The probe may also be dropped or fired into the formation for ballistic penetration. The probe may also be excited by a vibrator to create small, high frequency, vibratory displacements which result in partial fluidization of the formation material in direct contact with the probe. Such fluidization reduces the friction between the material and the probe surfaces, allowing it to sink into the bed sediments under the influence of its own weight. This invention is relevant to the latter vibratory method of penetration and is particularly applicable in achieving controlled penetration to relatively deep depths and/or under remote operating conditions, such as in deep water, where manned operation or diver access is impractical.
Known methods of sediment penetration typically provide penetration depths to about 20 feet. Hence, practical operation is possible by attaching a vibrator module in a fixed manner at the top end of the probe.
Thus, for probe lengths of 20 feet or less, full penetration may be accomplished without readjusting the vibrator position. For deeper penetration, the vibrator module may be clamped at intermediate positions on the probe riser pipe and moved upward as the probe penetrates into the sediments. In shallow water applications, this process may be accomplished by divers who adjust the vibrator position by means of a manual mechanical clamping arrangement. In deep water, a remotely operated clamping mechanism may be needed to replace this manual operation.
The sediment penetrating component may be a hollow pipe or tube when core samples are to be taken, and may have a solid, cylindrical form in the case of an instrumented probe. Such probes usually are attached to a hollow tube (riser pipe) of the same diameter to increase their effective length. If even greater length is required, additional riser pipes may be added.
A vibrator module is clamped onto the probe in such a way that strong vibrations may be imparted without slippage and without distorting or damaging the probe or tube. The clamping force required to attach the vibrator depends upon the dynamic force generated by the vibrator and the coefficient of friction at the clamping grip. The basic design objective and operational requirement of the vibrator and clamping mechanism is to mechanically drive the probe in longitudinal oscillatory motions having an amplitude in the range of approximately .+-.2 mm (.+-.0.08 in), or greater. This amplitude is sufficient to break much of the sediment contact friction on the probe surface.
The effective weight of the probe/vibrator assembly should be on the order of several hundred pounds in order to penetrate efficiently into underwater sediments. A typical vibrator mechanism might consist of two electric-motor-driven counter-rotating eccentric flywheels mounted in waterproof housings in a frame which positions the rotating masses on opposite sides of the cylindrical probe body. The eccentric rotors produce rotating centrifugal forces which, through the common mounting frame, seek their natural rotary synchronization within a few revolutions after being energized. Thereafter, the counter-rotating flywheels produce longitudinal oscillatory forces at the rotational frequency of the drive motors which, by proper clamping, can be coupled to the probe riser pipe.
The displacement of the vibrating probe is proportional to the longitudinal force imparted by the vibrator and inversely proportional to the mass of the probe/vibratory assembly and the square of the vibration frequency. Thus, for a given force (proportional to the vibrator power rating), the probe mass and rotational frequency must be designed or adjusted to produce the nominal .+-.2 mm (0.08 in.) displacement necessary for low-friction penetration. For example, the vibrator mechanism used in a recent prototype design was powered by two 2-hp electrical motors driving eccentric rotors that produced a peak oscillatory force of approximately .+-.7,000 lbf at a frequency of 3,600 rpm (60 Hz). For a probe/vibrator assembly mass of 300 lbs., the corresponding peak displacement was ##EQU1##
The penetration resistance of the probe can be made relatively independent of the penetration depth by designing the probe to have its lower end section larger in diameter than that of the upper riser pipe. With this design, only the larger diameter section of the probe governs the main penetration resistance and the probe can penetrate into the sediment to a depth limit governed primarily by the stiffness of the sediment material. This depth capability is estimated to be on the order of 100 feet for the 2-hp prototype vibrator probe system.
An object of the present invention is to provide a device that can be firmly clamped to a probe underwater, without the aid of a diver.
Another object is to provide a device which can be released, repositioned and reclamped on a probe from the surface in water too deep for safe diver operations.
Another object of this invention is to provide a clamping device combining high clamping force with simplicity, minimum size and weight, and with the low probe damage potential of collet-type clamps.